Embodiments of the invention relate generally to permanent magnet machines having high power-density and, more particularly, to a method and system for preventing fault conditions in a high power-density, high back electromotive force (emf) permanent magnet machines by providing power converters that include silicon carbide metal-oxide-semiconductor field effect transistors (MOSFETs).
The need for high power density and high efficiency electric machines (i.e., electric motors and generators) has long been prevalent for a variety of applications, particularly for hybrid and/or electric vehicle fraction applications. Due to energy supply and environmental reasons, there has been increased motivation to produce hybrid-electric and/or electric vehicles that are both highly efficient and reliable, yet reasonably priced for the average consumer. However, the drive motor technology available for hybrid-electric and electric vehicles has generally been cost-prohibitive, thereby reducing one (or both) of consumer affordability or manufacturer profitability.
Most commercially available hybrid-electric and electric vehicles rely on internal permanent magnet (IPM) electric machines for traction applications, as IPM machines have been found to have high power density and high efficiency over a wide speed range, and are also easily packaged in front-wheel-drive vehicles. However, in order to obtain such high power density, IPM machines must use expensive sintered high energy-product magnets. Furthermore, IPM machines run at high speed (e.g., 14,000 rpm) to obtain optimum power density. The power density of a permanent magnet machine is defined as the ratio of the power output and the volume of the permanent magnet machine. A relatively high power density (e.g., high power output relative to volume) is typically desirable. The high power density allows the permanent magnet machine to have either a smaller overall size for a given power output or a higher output for a given size.
As the speed of the rotor of the permanent magnet machine increases, the voltage developed in the stator (referred to as the “back emf”) increases. This, in turn, requires that higher and higher terminal voltages be applied to produce the desired torque. The machine back emf is proportional to speed for a permanent magnet machine. If the peak line-to-line back emf at maximum speed is higher than the DC link voltage, and if control over the power converter is lost, the permanent magnet machine will start operating in an uncontrolled generation (UCG) mode. UCG occurs when the control gate signals to all of the six inverter switches are turned off, or disconnected. During this condition, the motor is connected to the DC source via the anti-parallel diodes of the inverter switches. The anti-parallel diodes create a potential path for current to flow, which is dependent upon the motor operating condition and DC source voltage. In this case, the permanent magnet machine will act as a generator converting rotational power into electric currents and will start dumping energy into the DC link through the anti-parallel diodes in the power converter, causing an increase in the DC link voltage. If this energy is not dissipated, or if the build-up of the DC link voltage is not limited, the voltage rating of the active devices in the power converter may be exceeded by the DC link voltage.
In order to minimize or prevent occurrences of the UCG mode of operation, a limit is typically set on the machine back emf or an additional clamping or crowbar circuit is added in parallel to the DC link. However, limiting the machine back emf reduces the power or torque density and speed capacity of the machine. Further, adding a crowbar circuit adds additional cost and complexity to the circuitry of the permanent magnet machine drive system. The back emf of a machine can also be reduced by limiting the amount or relative strength of the magnets in the machine, which also negatively impacts the power or torque density.
It would therefore be desirable to eliminate setting a machine back emf limit and/or to eliminate adding a crowbar circuit such that device voltage ratings are not exceeded during a UCG mode of operation.